The present invention relates to alkaline electrochemical cells having porous cathodes comprising manganese dioxide.
Alkaline electrochemical cells have been commercially available for well over twenty years. In many modern applications, alkaline cells vastly outperform traditional zinc carbon cells, and are the batteries of choice for most consumers.
The competition to produce the best alkaline battery continues to be fierce, but a large number of restrictions exist, not least of which is the size of any given cell.
Conventional sizes for primary alkaline batteries include AAA, AA, C, D and 9V (LR03, LR6, LR14, LR20 and 6LR61, respectively), and standard dimensions are laid down for all of these types. Thus, whichever battery is chosen must fit within a given volume, thereby limiting the maximum amount of ingredients which it is possible to put into any given cell.
Working against these constraints, battery manufacturers have, for example, substantially reduced the thickness of the cell walls, reduced the thickness of the seals, and changed the nature of the labelling of the cell, all in order to optimise the use of the internal volume of the cell.
When it becomes impractical to further increase the volume of the cell ingredients, battery manufacturers then have the problem of trying to further enhance performance and battery life through enhancing and/or changing the ingredients used, but there must, ultimately, be a limit.
Another approach to improving battery discharge performance has been to increase the efficiency of active material utilisation during discharge. This is most effective for batteries intended for use under conditions where the discharge efficiency is low (e.g., in devices where the resistive load is low, the current drain is high, or the power consumption is high). This has become increasingly important as battery powered devices tend to place increasingly greater demands on the batteries.
In U.S. Pat. No. 5,283,139 ('139 or US '139 hereafter), there is disclosed a cell in which increased performance is achieved by increasing the density of both the anode and the cathode, without increasing the amount of aqueous, potassium hydroxide electrolyte. If the volume of a given active ingredient cannot be increased, then increasing its density is a logical, straightforward means for increasing the discharge capacity of the cell.
Nevertheless, there remains a desire to provide better and better electrochemical cells.
In International Patent Publication No. WO 01/99214, we demonstrate that, surprisingly, and contrary to expectations, substantial enhancement of cells, above and beyond those prepared in accordance with US '139, is possible, with performances being increased by as much as 15%, or more, by optimising the water ratios, in contrast to US '139.
Prior to '139, U.S. Pat. No. 5,489,493 disclosed alkaline electrochemical cells comprising a cathode comprising manganese dioxide, in which the cathode is composed of a mixture of a minor amount of highly porous manganese dioxide and a major amount of low porosity manganese dioxide. The highly porous manganese dioxide is exemplified as chemical manganese dioxide (CMD) and is distributed throughout the cathode in order to provide ion diffusion paths through the cathode. However, the use of CMD is not desirable in all cases, since CMD has both a lower peroxidation and a lower density than electrolytic manganese dioxide (EMD). Consequently, CMD has a lower theoretical capacity than EMD on a volumetric basis.
In WO 00/30193, there is disclosed the possibility of using a semi-solid cathode material. This semi-solid material has a high porosity and high electrolyte content, and primarily serves to reduce cathode polarisation effects. The drawbacks to this construction include the fact that there is a substantially reduced capacity in the cell and that the MnO2:C ratio is very low but, more importantly, that this cathode material becomes very difficult to handle, so that it is impractical to make a cell containing such material using conventional manufacturing processes and equipment. The capacity and performance of such cells is also severely compromised by comparison with US '139.
WO 98/50969 discloses the use of uniform zinc particles in the anode, which increases performances for anodes having porosities of up to and beyond 80%. Such porosities tend to separate zinc particles and increase impedance, especially in a 1-meter drop test. This disclosure teaches that increased porosities increase performance, provided that there is flaked zinc. High porosities are easily achievable, even at relatively high densities, as zinc is extremely dense, and no upper limit is indicated, although experimental data show good results between 75 and 80% porosity.
Battery industry standards traditionally used constant resistance tests to define battery performance levels for common types of applications. With changes in battery powered devices, together with the availability of more sophisticated testing equipment, there has been a trend to include constant current and, more recently, even constant power tests. There is now an increased importance on achieving improved battery performance on not only heavier loads, but on heavier loads to higher voltage endpoints under both constant current and constant power types of discharge. These trends have also contributed to an increasing need for batteries that perform well in niche areas, such as heavy load, constant current (Amps), constant power (Watts), continuous, and intermittent discharge, and combinations thereof.
Raising battery voltage on discharge can improve the discharge capacity on heavy drain discharge, particularly to higher voltage endpoints. In alkaline batteries, the use of voltage boosting agents such as Ag2O and ferrates is theoretically attractive, but cost, handling, and instability can be problems. Therefore, lowering the internal resistance is the only practical way to increase the operating voltage of the battery. Efforts to increase the electrical conductivity of the anode and cathode as a means of improving discharge performance at high constant wattage have met with little or no success, as the necessary changes in other parameters has counteracted the improvements.
It is well known that dry cell batteries were originally made with mercury, but concern about potential damage to the environment caused by the disposal of large amounts of mercury-containing batteries was substantial. Thus, the amount of mercury contained in dry cell batteries has now been reduced to the extent that, in most consumer batteries on the market, there is no added mercury. There may be traces of mercury present in the zinc, but levels are measured in parts per million.
Reducing the amount of mercury was no easy feat as, apart from anything else, it served to prevent gassing. Removal has proven possible by reducing gassing in other ways, such as improving the purity of the various electrode and electrolyte constituents and by adding various additives, such as substituted ammonium derivatives.
With the elimination of mercury, increasing the levels of zinc also proved necessary, because mercury also contributed to the conductivity of the anode. Removing mercury made the anodic zinc electronically inefficient, and the quantity of zinc had to be increased. To maintain the electrical conductivity at a sufficiently high level throughout discharge, an excess of zinc is generally required. A ratio of zinc capacity to cathode capacity of about 1.33:1 is common. However, the function of the excess zinc is that of a conductor, not an active material, so it limits the amounts of active material that can be put into the battery. An additional disadvantage is that any zinc left after the cell has been exhausted can react with any remaining water to create hydrogen.
The 1.33:1 anode:cathode ratio is common, at least partially, because of the second electron reaction, where MnIII→MnII. This reaction is secondary to the primary reaction in which MnIV→MnIII, but has been considered significant in the total discharge capacity of cells, to date. The second electron reaction takes place in solution at appropriate centres, which tend to be the graphite used for creating conductivity in the cathode. Graphite is not an active material, so the quantity used is generally reduced as far as possible, with a ratio of at least 20:1 active manganese dioxide:graphite being common. Greater ratios of active manganese dioxide:graphite are constantly being sought, in order to maximise the active material, while maintaining conductivity.
However, reducing the quantity of graphite reduces the active centres for conducting the second reaction, so that the second electron reaction becomes inefficient. Accordingly, less of the zinc is consumed, leaving some to gas after cell failure. This effect leads to leakage following deep discharge. Because reducing zinc levels to compensate leads to lower cell discharge capacity, a better option is to ensure that cells will not vent or leak as a result of the typical internal pressures that build up under non-abuse situations.